Asynchronous Programming doesn’t imply multithreading!


This is a part of the blog series , “Asynchronous Programming Using DotNet” .Please visit this page to see the “index” of all the posts I wrote on this series.

Few days ago,while talking to some of my friends,I realized a lot of people harbor a misconception that implementing asynchronous models makes their program multithreaded and parallel .I will try and debunk this myth in this post.

People often think asynchronous work equal multithreading. Which is absolutely not true?

In this post we are going to differentiate asynchronousness and parallelism.

Now let’s take an example, say we are consuming a web service or using the web Client which involves a call over the network and significant waiting time. The first thing that comes to our mind when we do such a work is that we must keep the UI responsive while our program waits. A lot of people think that to keep the UI responsive there HAS to be a separate thread which is running and taking care of our web request.

But the truth is different. The request only needs two threads at a specific point. It’s also possible to do the work in ONE thread without blocking the thread or freezing up the UI.

I will back up my claims logically

1. Most of the time is spent waiting for a response from the server. When a client machine sends a request, it has to wait for the reply. So what would a thread do when the request is in progress? If we use synchronous programming model, it will put the thread to sleep as there is no work for the thread to do.

2. Even while the client computer, is sending or receiving data /request to the server we do not need a different thread as this is taken care of by the network hardware. The network hardware inside the computer is quite capable to handle stuffs related to such tasks. When we actually send the data, the device driver for the NIC, programs the hardware pointing it to the data to send. The network hardware in most cases is quite capable of fetching the actual data to be sent directly from the main memory. So the driver only needs to inform the location of the data in the memory. So the work of CPU is only limited to telling the NIC where to fetch data from and where to send the data. The time taken to do that is quite negligible compared to the time taken to send the data.

3. The above case is true for most Input/output tasks. It’s the same with disk access. The CPU just needs to tell the disk controller as to which data needs to be accessed. The time is miniscule compared to the time disk controller takes to move its parts and access the data. The time the CPU takes to issue the instruction is very small compared to the above.

Practically most windows applications spend a lot of time waiting .Multithreading won’t make the “waiting” faster. It’s the asynchronous ness that would keep the program responsive. Multithreading is important when there is a specialized background that is running. That is when a multicore benefit will be more pronounced.

The main point of asynchronous code is mostly to reduce the number of threads we are using. It does that by taking away threads from code that may have blocked it. It will only be allowed to consume threads at only those moments when it actually has constructive work (useful CPU work) to do. This is what leaves the UI thread free to respond to the user input

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One response to “Asynchronous Programming doesn’t imply multithreading!

  1. Pingback: Asynchronous Programming Series « Using Abhik.Mitra.myThoughts;

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